1. Field of the Invention
The present invention relates to a system and method employing concatenated spreading sequences to provide data modulated spread signals having increased data rates with extended multi-path delay spread. More particularly, the present invention relates to a system and method employing concatenated spreading sequences to remove the one-to-one relationship between the repetitive spreading code length and maximum multi-path delay spread of a data modulated spread signal, to thus increase the data rate for a given multi-path delay spread while preserving the autocorrelation and cross-correlation properties of the individual spreading sequences.
2. Description of the Related Art
Many communications systems, such as wireless terrestrial or satellite-based communications networks, often employ spread spectrum modulation techniques to render the communications signals less susceptible to noise, interference, and multi-path channel effects. As can be appreciated by one skilled in the art, a transmitter employing a spread spectrum technique typically uses a sequential noise-like signal structure, such as pseudonoise (PN) codes, to spread a normally narrow-band information signal over a relatively wide-band of frequencies during transmission. A receiver despreads the signal to retrieve the original information signal by correlating the received spread spectrum signal with the known PN code waveform.
Known spreading techniques typically use a single repetitive spreading sequence to spread the data being transmitted, which results in a one-to-one relationship between the spread code length and maximum multi-path delay spread. A single repetitive spreading sequence is typically used for the synchronization section of the waveform to reduce hardware complexity introduced by the sliding matched filter implementations. To preserve the autocorrelation properties of the spreading code, a single data symbol is spread across an entire non-repetitive section of the spreading code, which limits the maximum data rate to the required multi-path delay spread for the system. The data symbol consists of 1 bit for biphase shift keying (BPSK) data modulation, 2 bits for quadrature phase shift keying (QPSK) modulation, and M bits for higher bit modulation formats.
FIG. 1 illustrates an example of a 128 chip repetitive spreading sequence, identified as xe2x80x9cspreading code 1xe2x80x9d , including chips c1(1) through c1(128), being modulated by a data stream including data symbols D1 through D4, to provide a multi-path delay spread of xc2x1128 chips. This modulation limits the data symbol period to 128 chips for spreading across one repetitive section of the spreading code. As indicated, the modulation produces a direct path signal, including modulated symbols D1*c1(1) through D1*c1(128), D2*c1(1) through D2*c1(128), and so on. FIG. 1 further illustrates the relationship between this direct signal path and a multi-path signal generated by the communications channel that is delayed by 31 chips with respect to the direct path signal.
As further illustrated, the received signal is the combination of the direct and multi-path signal, which introduces both constructive and destructive chip interference. An example of the effect of constructive and destructive chip interferences is also illustrated for BPSK data symbols of +1 and xe2x88x921. Similar constructive and destructive chip interference exists for higher modulation formats. Since interference exists on a chip level versus a symbol level, spreading sequences with good autocorrelation properties (i.e., low sidelobe levels) are able to recover the original signal. Accordingly, by using a Rake receiver, a receiver is able to recover both the direct and multi-path signal by despreading with two properly time aligned spreading sequence structure as shown in FIG. 2.
It is further noted that the delay between the multi-path signal and the direct path signal can be increased to, for example, 128 chips, as shown in FIG. 3. The received signal resulting from this relationship is also shown in FIG. 3. As indicated, instead of the multi-path signal introducing chip interference, symbol interference is introduced by this chip delay. That is, because the multi-path signal is delayed by 128 chips, the receiver receives the modulated symbols D1*c1(1) through D1*c1(128) of the multi-path signal at the same moments in time that it receives the modulated symbols D2*c1(1) through D2*c1(128) of the direct path signal. Likewise, the receiver receives the modulated symbols D2*c1(1) through D2*c1(128) of the multi-path signal at the same moments in time that it receives the modulated symbols D3*c1(1) through D3*c1(128) of the direct path signal, and so on.
As further indicated in FIG. 3 for BPSK data modulation, if the symbols D1 and D2, for example, are equal, then the received signal experiences constructive symbol interference between the modulated symbols D1 and D2 which are received at the same moments of time as discussed above. However, if the symbol D1 is equal to xe2x88x92D2, for example, then the received signal experiences destructive symbol interference between the modulated symbols D1 and D2. Both the constructive and destructive symbol interference extends across an entire repetitive spreading sequence as indicated. If destructive interference of the type described above occurs, recovery of the symbols is impossible, because the destructive interference reduces the symbol information to zero. However, it can be appreciated that if the multi-path delay is increased beyond 128 chips, the recovery of data symbols is again possible, because only chip interference, not symbol interference occurs. Similar constructive and destructive symbol interference exists for higher modulation formats.
In order to prevent the potentially disastrous affects caused by destructive interference, it is typical for know systems to limit the multi-path delay spread to the maximum length of the spreading code. Hence, if a multi-path delay spread of 128 chips is desired, the maximum data period is also limited to less than 128 chips. In this event, in order to double the data rate without changing the modulation format, it is necessary to reduce the data period to 64 chips. Since the multi-path delay spread is related to the data period by a one-to-one ratio, the multi-path delay spread would also have to be decreased to less than 64 chips.
Accordingly, a need exists for a system and method which enables a transmitter employing signal spreading techniques to increase the data rate of the transmitted modulated signal without reducing the multi-path delay spread significantly.
An object of the present invention is to enable a transmitter employing signal spreading techniques to increase the data rate of the transmitted modulated signal without reducing the multi-path delay spread.
Another object of the present invention is to enable a transmitter employing signal spreading techniques to increase the data rate for a given multi-path delay spread while preserving the autocorrelation and cross-correlation properties of the individual spreading sequences at which the data symbols are spread.
Another object of the present invention is to enable a receiver to employ a Rake receiver structure to enhance multi-path performance.
These and other objects are substantially achieved by providing a system and method for spreading a modulated data sequence using concatenated spreading sequences. The system and method enables a transmitter to generate spreading code sequences, and to create a plurality of concatenated spreading sequences, each comprising a plurality of the spreading code sequences concatenated together and having a length no greater than a predetermined maximum length. The transmitter then spreads each of the data symbols with one of the respective concatenated spreading sequences to produce a modulated signal, representing the direct path signal at the receiver. The system and method further enable the receiver to despread the direct path and multi-path signals using an a priori knowledge of the concatenated spreading sequences in combination with a RAKE architecture which despreads each multi-path RAKE tap. Accordingly, the system and method removes the one-to-one relationship between the repetitive spreading code length and maximum multi-path delay spread of a data modulated spread signal, to thus increase the data rate for a given multi-path delay spread and modulation format while preserving the autocorrelation and cross-correlation properties of the individual spreading sequences.